It is well known that in order to turn on and off high power switching devices, such as IGBT/MOSFETs, employed in various power electronic converters the on-off control signals need to be electrically isolated from the main power switch. The electrical isolation is typically accomplished by means of photo-couplers. The resulting isolated control signal is amplified using an isolated power supply before being applied across the Gate and Emitter junction of the IGBT/MOSFET. Lately, to reduce component count and cost, the photo-couplers take on the task of electrically isolating as well as amplifying the signal, which is then applied directly across the Gate and Emitter junction of IGBT/MOSFETs.
The above mentioned method, which is currently used by a majority of power semiconductor users, is not well suited for high frequency applications in the hundreds of kilo hertz (kHz) range. Many Variable Frequency Drive (VFD) manufacturers are working on bringing out power semiconductor switches employing Silicon Carbide and/or Gallium Nitride devices that have low switching losses and hence can be turned ON and OFF at hundreds of kHz. Higher switching speed using devices that are known to have reduced power loss will enable to operate motors at higher speeds and reduce the VFD foot print per kW of delivered output power. External filtering component sizes can also be dramatically reduced. In response to such positive research activity, it is important to consider efficient means of turning ON and OFF the future power semiconductor switches that will be employing GaN devices.
Since existing IGBT/MOSFET switches as well as the future switches will be voltage controlled devices, the Gate to Emitter junction will have a metal-oxide layer, which will be associated with an input capacitance. Turning ON and OFF these devices entails charging and discharging the junction capacitance. When the rate at which the junction is turned ON and OFF is increased, the current consumed by the gate circuit will naturally increase. Higher current entails a larger gate drive power supply and possibly more gate drive power loss.
At higher operating speed, the delay introduced by the intrinsic capacitance of the photo-coupler will be noticeable and can create unfavorable control response.
Many researchers have proposed resonant gate drive circuits to reduce power consumption in gate drive circuits and to improve the speed at which Si based semiconductor switches can be turned ON and OFF.
One of the earliest researchers on this topic, Dr. Robert Steigerwald, has published many articles on this subject, and has at least two patents that are relevant here. In U.S. Pat. No. 4,967,109, a resonating inductor is used in series with a switch in the gate-emitter circuit. The inductor resonates with the gate-emitter capacitor thereby charging the capacitor to a value close to twice the gate drive power supply voltage. During the first half of the charging cycle, energy is stored in the inductor, which is then released into the gate-emitter capacitor in the latter half, thereby boosting the capacitor voltage as mentioned. A diode in series with the switch is used to prevent the reversal of the resonant current after it goes through its natural zero. Since the switch and the blocking diode carry the resonant charging current, the power loss in these devices cannot be neglected. Moreover, the voltage across the inductor needs to be reset, using some external means, which is not shown in the patent, so that the inductor is ready for the next turn ON pulse. Due to the presence of the switch and diode in series with the inductor, such a circuit is not well suited for high frequency operation.
At turn off, in the above circuit, the gate-emitter capacitor is simply discharged into the source via a switch. Instead of wasting this energy, the same author, in U.S. Pat. No. 5,010,261, used an external capacitor to transfer the gate energy on to this external capacitor. The transfer is facilitated by an external inductor and a bidirectional switch. Theoretically, the arrangement in U.S. Pat. No. 5,010,261 should result in a very low loss gating scheme. However, due to the presence of the bidirectional switch in addition to the main gate drive switch, the overall loss is not significantly reduced. In addition, the component count has increased, which makes the circuit in U.S. Pat. No. 5,010,261 unattractive.
A more complex scheme utilizing a high frequency transformer in place of an inductor is used for storing the discharge energy of the capacitor in the circuit proposed in U.S. Pat. No. 5,134,320. Because of the added external circuit components and the power loss associated with such external devices, the circuit proposed in U.S. Pat. No. 5,134,320 does not meet the requirements for the suggested application.
A resonant gate drive circuit utilizing coupled inductors and external diodes to return part of the stored gate energy back to the gate drive power supply forms the core of the circuit proposed in U.S. Pat. No. 5,264,736. Again, the fact that charging current is made to flow through driver transistors and external diodes make this circuit bulky, less efficient, and more expensive.
A circuit quite similar to the one in U.S. Pat. No. 4,967,109 has been proposed in U.S. Pat. No. 5,804,943 and suffers from similar disadvantages.
When an inductor is used as part of a gate drive scheme to charge up the gate-emitter capacitance of an IGBT/MOSFET, as in all the circuits discussed thus far, there is a cross conduction problem that entails power loss, when the upper driver transistor is turned OFF and the lower transistor is turned ON to discharge the capacitor. In order to avoid such occurrence, either a cross current regulating inductor of the type mentioned in U.S. Pat. No. 5,264,736 can be used or one could use two separate inductors as mentioned by Ian D. de Vries, in “A Resonant Power MOSFET/IGBT Gate Driver”, IEEE Applied Power Electronics Conference and Exposition (APEC) 2002, Vol. 1, pp 179-185. The circuit discussed therein has been operated at a carrier frequency of 1 MHz where the resonating inductor is formed by simply using a wire of length 6 cms. However, in all circuits that force the complete charging current through the inductor and a switch are always going to have non trivial power loss associated with them.
One known circuit advocates parallel paths for charging the gate-emitter capacitance. A resonant inductor path provides the initial charging current to quickly charge the gate-emitter capacitance. The remaining current needed to bring the gate-emitter voltage of the IGBT/MOSFET to the Vcc level of the supply voltage is provided by an alternate low loss path. This basic idea forms the core of the Japanese patent 3-60360.
The circuit documented in Japan patent 3-60360 is shown in FIG. 1. The basic operation involves charging the gate-emitter capacitance C by an initial surge of current provided by the energy exchange between the resonating inductor L and the gate-emitter capacitance. After the gate-emitter capacitance C gets charged to a certain turn-on level, the complete turn ON is achieved by augmenting the gate current from the standard gate power supply. This forms a two stage charging strategy that reduces the current rating of the photo coupler and improves the propagation delay. The energy stored in the gate-emitter capacitance is basically circulated between the resonating inductor and the capacitance. During this circulation there is some amount of loss, which needs to be replenished from the gate power supply. The overall strategy results in optimal performance and reduces the gate power requirements.
Referring to FIG. 1, it is assumed that the voltage across the gate-emitter capacitance C is at −Vcc to begin. When it is desired to turn on the IGBT/MOSFET Q5, a turn ON signal is provided to a switch 111 (the switch in series with the resonating inductor L, the entire combination of which is in shunt, across the gate-emitter of the main IGBT/MOSFET Q5). A resonating current pulse enables the transfer of energy stored in the gate-emitter capacitance C to the resonating inductor L. In the process, the gate-emitter capacitor C charges from −Vcc to a certain positive value which is close to +Vcc but not exactly at +Vcc due to losses in the circuit. The voltage across the gate-emitter is brought up to +Vcc by turning ON switch 61D, which connects the gate terminal to +Vcc of the gate power supply via the charging resistor R. The second charging stage ensures that the main IGBT/MOSFET operates in the saturation state.
When it is desired to turn OFF the main IGBT/MOSFET Q5, a turn off signal is given to a switch 112 and a resonating current pulse flows through the gate-emitter capacitance C and inductor L in the opposite direction and removes the charge from the gate thereby enabling safe turn off. The gate-emitter capacitance C reverses in polarity and a second discharge state to −Vcc is provided by turning ON a switch 62D after a brief moment.
The key to proper operation of the circuit in FIG. 1 is that the switches 61D and 62D connecting the gate to +Vcc and −Vcc, respectively are delayed compared to the switches 111 and 112 that allow transfer of energy between the resonating inductor L and the gate-emitter capacitance C. A circuit that achieves this requirement is shown in FIG. 2 and is part of the Japan patent 3-60360. The buffer amplifier shown in FIG. 2 is generic and any commercially available part can be used as the buffer amplifier.
The delay in turning ON of switches 61D and 62D is provided by the RC circuit in the base drive of these transistors formed by a resistor 18 and a capacitor 17. In order to reduce the power loss in transistors 111 and 112, these are turned ON for only a short duration of time, enough to facilitate the transfer of energy between the resonating inductor L and the gate-emitter capacitance C of the main IGBT/MOSFET Q5. Hence, the base drive for transistors 111 and 112 have an R-C differential circuit formed by a resistor 14 and a capacitor 12.
The circuit implementation shown in FIG. 2 does not show that the main pulse used for turning ON and OFF the power IGBT/MOSFET Q5 is in fact isolated (either optically or magnetically) from the control circuit that generates the pulse. It is assumed that one knowledgeable in this area understands the need to isolate the control logic from the gate drive circuit. However, the circuit in FIG. 2 can be modified to provide an easier implementation using a traditional photo coupler.
The present invention is directed to further improvements in resonant gate drive circuits. Particularly, in order to reduce the ON and OFF delay, as well as reduce the overall propagation delay, improvements to the circuit shown in Japanese patent 3-60360 are disclosed herein.